Fatal error: Unable to bridge NSNumber to CGFloat: file /BuildRoot/Library/Caches/com.apple.xbs/Sources/swiftlang/swiftlang-902.0.48/src/swift/stdlib/public/SDK/Foundation/NSNumber.swift, line 606

import UIKit

class SampleLayer: CALayer {
    var brightness: CGFloat = 0
    
    @objc dynamic var locations: [CGFloat] = [0.0, 1.0] {
    didSet {
        setNeedsDisplay()
    }
    }
    
    func setBrightnessAnimated(_ brightness: CGFloat, duration: CFTimeInterval, timingFunction: CAMediaTimingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseOut)) {
        let oldBrightness = self.brightness
        let newBrightness = brightness
        
        // On iOS 9 with Swift 4.1 this is crashing due to a bug where CGFloat cannot be cast to NSNumber in some conditions
        if duration > 0 {
            let oldLocations = presentation()?.locations ?? [0, oldBrightness]
            let animation = CABasicAnimation(keyPath: "locations")
            animation.duration = duration
            animation.fromValue = oldLocations
            animation.timingFunction = timingFunction
            removeAnimation(forKey: "brightness")
            add(animation, forKey: "brightness")
        }
        
        self.brightness = newBrightness
    }
}

let testLayer = SampleLayer()
testLayer.setBrightnessAnimated(1, duration: 0.1)